“They are like a dream when one wakes up.” There is no real substance to the wicked. When Yahweh awakes, there will be nothing to them and they will disappear.
“when aroused.” So much of the time today, it seems as if God is asleep. The wicked prosper and the righteous suffer. But when God does wake up—and He will—all the dreams and aspirations of the wicked will be seen to be just fantasies, and they will be gone like a dream is gone in the morning.